Woo who (3)

[Tony Jones]

Brian Winch seems to have been stung by comments on this forum (though he doesn't mention it specifically) and emails he has received about various topics on which he has recently written.

 

In his column in the March issue he states 'I certainly do know and understand the laws put forward by Sir Isaac Newton and have used them in many ways thus far in my life'.

 

The then establishes - quite rightly - that a RC model in flight is never in a steady state but constantly subject to a variety of accelerations in all three planes. But from this he draws the conclusion: '...that 's why the fuel is almost always at or moving toward the rear of the tank'. IMHO - and I emphasise the 'H' - that would only be true if the model was subject exclusively to forward acceleration, which it clearly isn't.

 

Please feel free to shoot me down if I've got this wrong. I always try to learn from such experiences.

 

Best wishes

[Martyn K]

I think that is true, it also applies to the fuel in the fuel feed pipe less any capilliary attraction, which I suspect would be minimal.

 

Take the case where the model is climbing vertically, gravitational acceleration would ensure that fuel is being forced to the rear of the tank. However EXACTLY the same force is being applied to fuel in the feed pipe.

 

This is why full size cars, boats, aircraft have fuel pumps - to guarantee a consistent supply. We (or most of us) rely on pressure (albeit minimal) from the exhaust system and the venturi effect causing an area of very low pressure in the carburretor, (which encourages fuel to flow from a volume that is subject to a higher pressure - the tank) neither of which actually guarantee a consistent supply. (How many people have dead sticks with plenty of fuel in the tank?)

 

I believe that this is the reason why top aerobats use (or used to use) fuel pumps (I am not sure if that is still the case), contest freeflighters and C/Liners use crankcase pressure (or pacifier tanks) - typically higher than exhaust pressure to help guarantee the supply.

 

But then again I may be totally wrong..

 

Martyn

Edited By Martyn K on 08/02/2012 17:31:58

[Tony Jones]

Posted by Martyn K on 08/02/2012 17:31:12:

I think that is true, it also applies to the fuel in the fuel feed pipe less any capilliary attraction, which I suspect would be minimal.

 

Martyn

Edited By Martyn K on 08/02/2012 17:31:58

 

But would you agree with the statement 'the fuel is almost always at or moving toward the rear of the tank', when the model is in level flight at a steady throttle setting?

 

Regards

[Tony Jones]

Posted by Martyn K on 08/02/2012 17:31:12:

I think that is true, it also applies to the fuel in the fuel feed pipe less any capilliary attraction, which I suspect would be minimal.

 

Martyn

Edited By Martyn K on 08/02/2012 17:31:58

 

Oh botheration. must have double clicked again.

 

Regards

Edited By Tony Jones on 08/02/2012 19:48:21

[Ben B]

I think the only way to sort this out is a keychain camera, a LED light source and a plane

[John Privett]

Posted by Tony Jones on 08/02/2012 19:46:59:

 

But would you agree with the statement 'the fuel is almost always at or moving toward the rear of the tank', when the model is in level flight at a steady throttle setting?

 

 

No, absolutely not.

 

When the model is accellerating or flying at a constant speed with the nose raised (climbing perhaps) then the fuel will tend to be at the back of the tank. When it's decellerating or flying at a constant speed with the nose lowered then the fuel will tend to be at the front of the tank. And when turning the fuel will be towards the side of the tank that is on the outside of the turn. In constant-speed, level flight the fuel will sit in the tank much as it does when the plane is on the ground. Admittedly our models are rarely on a constant heading at a constant speed, but deviations from this will have the fuel moving all over the place and not "almost always at or moving towards the back of the tank."

 

I know he tries to dismiss the "drinks onboard an airliner" analogy with some spurious excuses, but that analogy is sound. Whilst hurtling through the sky at 550mph, the coffee in our Sleazyjet paper cup - just like the fuel in our models - is not 'almost always at or moving toward the rear of the tank'. We'd have wet shirts if it was.

 

[Stephen Grigg]

I have seen a video of what happens to fuel in a tank in flight,goes everywhere!!!thats why we need the clunk

[Martyn K]

It is very easy to test the theory. Get a clunk fuel tank, add a short feed pipe and 1/2 fill it with blackcurrant juice so you see what is going on.

 

Move it upwards. If Mr Newton is wrong, then blackcurrant will flow out of the fuel pipe which is currently pointing upwards.

 

Hold it level arms length and start to spin. You will initially see the fuel flow to the back of the tank and then when you reach a steady velocity, gravity will take over and the level will stabilise. When you slow down again, the fuel will flow to the front of the tank. Even when you are accelerating hard, there should be no blackcurrant coming out of the pipe. It will simply find a common level (discounting a tiny bit of capilliary action) with the level in the tank matching the level in the pipe.

 

Try it ...

 

And just to settle another point in this months Physics lesson. When a model is flying straight and level, the 4 forces competing against eah other, Thrust v Drag and Lift v Mass are in equilibrium. Instantaneously, the model is at rest. When we stand on the earth, we have two forces, gravity pulling us down and the inherent strength in our body preventing us from collapsing down to a film of only a few atoms thick.. We are also in equilibrium. There is also a 3rd force - centrifugal trying to chuck us into space, but gravity dominates.

 

When the model is in equilibrium, it is not accelerating or decelerating nor is is it climbing or sinking. In this case the fuel in the tank (as a liquid) is being pulled solely by the force of gravity and will settle on a true level.

 

 

Martyn

 

 

Edited By Martyn K on 08/02/2012 23:43:07

[Martyn K]

 

Posted by Stephen Grigg on 08/02/2012 21:36:37:
I have seen a video of what happens to fuel in a tank in flight,goes everywhere!!!thats why we need the clunk

What is happening in reality is that there are vibrations etc which gets the fuel vibrating (in harmony plus harmonics). Of course, the clunk is required so you can fly knife edge or upside down.

[Martyn K]

To be fair to the WOO, he is absolutely correct that glow fuels are hydrophilic.

 

M

[Bob Cotsford]

but he goes on to talk about 60% water content - I think that's stretching the bounds a bit in practise.

Edited By Bob Cotsford on 09/02/2012 00:10:56

[Tony Jones]

 

I know he tries to dismiss the "drinks onboard an airliner" analogy with some spurious excuses, but that analogy is sound. Whilst hurtling through the sky at 550mph, the coffee in our Sleazyjet paper cup - just like the fuel in our models - is not 'almost always at or moving toward the rear of the tank'. We'd have wet shirts if it was.

 

Yup. The reason the clunk extends to the back of the tank is nothing to do with the fuel being at the back of the tank in level flight - which it isn't. It's because if it was anywhere else the engine would cut when the fuel does go to the back of the tank - which is when the plane is climbing. Which IMHO is part of the reason for holding a model that hasn't been flown that day 'nose high' - not only to check that the mixture isn't too lean but that the clunk hasn't got stuck up the front of the tank.

 

I'm really amazed that anyone can claim strenuously that they DO understand physics when the clearly don't.

Regards

Edited By Tony Jones on 09/02/2012 00:31:34

[Martyn K]

Posted by Bob Cotsford on 09/02/2012 00:10:40:

but he goes on to talk about 60% water content - I think that's stretching the bounds a bit in practise.

Edited By Bob Cotsford on 09/02/2012 00:10:56

I hadn't spotted that... I think that there is a misunderstanding. Methanol is infinitely soluble in Water. I presume therefore that is it feasible in a very wet environment for the methanol to be fully absorbed (like the bottom of a full swimming pool). The reality is that will not happen of course, but it you leave the lid off your fuel bottle in a damp environment for a week, moisture will be absorbed into the fuel and your fuel will be shot (he says from experience). You will also lose other constituents of the fuel through evaporation. However, if you are stupid (or do not learn from your experience) enough to do that you deserve to have a very erratic running engine that will start, then stop when it feels like it and be confident that it has nothing to do with the location of your fuel tank.

 

M

[Martin Whybrow]

He also seems to be confused about what a tappet is, he refers to the rocker arms as tappets; I've always understood the tappets to be the cam followers, not the rockers. I know they are one and the same for some OHC engines, e.g. the Ford Pinto engine, but it's not generally the case.

[Tony Jones]

Posted by Martin Whybrow on 12/02/2012 19:12:53:
He also seems to be confused about what a tappet is, he refers to the rocker arms as tappets; I've always understood the tappets to be the cam followers, not the rockers. I know they are one and the same for some OHC engines, e.g. the Ford Pinto engine, but it's not generally the case.

I'd agree with that. And with pushrod engines the clearance is adjusted with the set screw in the rocker arm. I wish I had 1p for the number I've adjusted on cars in my time...

 

Best wishes.

[Myron Beaumont]

Has anyone ever noticed how quickly an air bubble disappears in the fuel line from tank to carb? Makes me think that peeps with fuel flow problems (3D flyers for example )should maybe incorporate several loops of fuel tubing in said connection to overcome any small time lack of fuel situations.In other words ,any situation where the clunk is not surrounded by neat fuel or froth will not interrupt the running of the engine enough to stop it

[Bob Cotsford]

Now I'd always understood the tappets to be the adjuster screws on pushrod engines - ie the bit you adjust as in 'I've adjusted the tappets'. I've used that term for more than 40 years since my Dad showed me how to adjust the tappets on my first BSA Gold Flash.

[Myron Beaumont]

Bob

Never heard of a Gold Flash .Don't you mean Gold Star ?

I think the word tappet comes from Tap it -the sound emanating from the regular contact with the rocker arm with the valve stem

[Martyn K]

The Golden Flash was the ubiqutous A10 from BSA. A decent one is worth a fortune now.

 

Martyn

[Tony Jones]

Posted by Bob Cotsford on 12/02/2012 22:39:19:

Now I'd always understood the tappets to be the adjuster screws on pushrod engines - ie the bit you adjust as in 'I've adjusted the tappets'. I've used that term for more than 40 years since my Dad showed me how to adjust the tappets on my first BSA Gold Flash.

 

I think you'll find that the tappets are actually the followers that ride on the cam lobes. There has to be some clearance in the 'pushrod train' and the way it's adjusted is the way you say. In fact, the term 'adjusting the valve clearance' is a more accurate way of describing it. Wikipedia says tappet is 'the noise made by a worn cam follower' but I 'm afraid I don't agree.

 

Regards

[Tony Jones]

Posted by Bob Cotsford on 12/02/2012 22:39:19:
Now I'd always understood the tappets to be the adjuster screws on pushrod engines - ie the bit you adjust as in 'I've adjusted the tappets'. I've used that term for more than 40 years since my Dad showed me how to adjust the tappets on my first BSA Gold Flash.

Here's a diagram I found...

 

http://dl.dropbox.com/u/37947215/flat-tappet-engines.jpg

 

Best wishes.

[Bob Cotsford]

on the other hand, from Wikipedia:

 

A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly. Properly speaking, a tappet is the only part of a rocker arm that makes contact with an intake or exhaust valve stem above the cylinder head of an internal combustion engine. As the cam rotates, it creates both a sideways and a downward force on the tappet. Without a tappet (and with the cam acting directly on the valve), the sideways force would cause the valve stem to bend. With a tappet, the sideways force is transferred to the cylinder head so only the downward force acts on the valve stem.

 

So I guess it's a pretty vague term defined principally by customary usage and exactly where you look up the definition.

[Tony Jones]

I would venture that in this case Wikipedia has got it wrong. My experience is that professional engineers specialising in four-stroke engines always refer to the actual cam follower as the tappet - as per the diagram I posted a link to. Not the adjusting screw that is part of the rocker arm and is used to adjust the valve clearance on overhead valve engines.

 

It's also wrong in stating that : Without a tappet (and with the cam acting directly on the valve), the sideways force would cause the valve stem to bend. On an overhead camshaft engine like the Jaguar XK series the cam lobe DOES act directly on the valve stem head via some shims. Changing these shims is how the valve clearance is set. And it's a right royal pain as the camshaft has to removed each time and you very rarely get it right first time. Don't ask me how I know this.

 

Best wishes

 

Regards

[Martin Harris - Moderator]

I'm well aware that this terminology has been the subject of discussion in the automotive world for as long as I've been wielding spanners and no doubt since a long time before!

 

All I'd say is that I've never seen a tappet in a parts list and the usually accepted term where it's used is "adjusting the tappet clearances" which might fit in reasonably well with the Wikipedia description if we disregard the worn aspect - there will always be some noise generated. The part referred to in Tony's post is generally listed as a cam follower in my collection of parts books (to the best of my recollection).

[Bob Cotsford]

Doesn't the cam in an ohc engine act on a bucket, which both holds the shims and takes all the side loads?

 

edit - apart from the old Cavalier/Astra etc engine where the OHC cam acted through a rocker which wore out every 60k miles.

Edited By Bob Cotsford on 13/02/2012 13:21:00